Clarify GraphicsContext::beginLayer()/endLayer() have unexpected behaviors.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

Index: Source/core/html/canvas/CanvasRenderingContext2D.cpp
diff --git a/Source/core/html/canvas/CanvasRenderingContext2D.cpp b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
index a1911e5990147f992e81a3511e0192d35fa2bb4a..4f8f1691ce6c16f473347fbe96058292454d7c84 100644
--- a/Source/core/html/canvas/CanvasRenderingContext2D.cpp
+++ b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
@@ -1564,6 +1564,7 @@ void CanvasRenderingContext2D::drawImageInternal(CanvasImageSource* imageSource,
             didDraw(dirtyRect);
         }
     }
+    validateStateStack();

[dshwang, 2014/10/14 19:00:28]

I move it to here, because drawVideo is used as follows:
c->beginLayer(1, op);
drawVideo();
c->endLayer();

c->beginLayer(1, op) slightly increase state stack while
CanvasRenderingContext2D doesn't know it.

It's a bug. I'll fix it.

 
     if (sourceImageStatus == ExternalSourceImageStatus && isAccelerated() && canvas()->buffer())
         canvas()->buffer()->flush();
@@ -1582,7 +1583,6 @@ void CanvasRenderingContext2D::drawVideo(HTMLVideoElement* video, FloatRect srcR
     c->translate(-srcRect.x(), -srcRect.y());
     video->paintCurrentFrameInContext(c, IntRect(IntPoint(), IntSize(video->videoWidth(), video->videoHeight())));
     stateSaver.restore();
-    validateStateStack();

[Justin Novosad, 2014/10/14 17:50:25]

why are you removing this?

 }
 
 void CanvasRenderingContext2D::drawImageFromRect(HTMLImageElement* image,
@@ -1647,10 +1647,8 @@ template<class T> void CanvasRenderingContext2D::fullCanvasCompositedFill(const
     GraphicsContext* c = drawingContext();
     ASSERT(c);
     c->beginLayer(1, state().m_globalComposite);
-    CompositeOperator previousOperator = c->compositeOperation();
     c->setCompositeOperation(CompositeSourceOver);
     fillPrimitive(area, c);
-    c->setCompositeOperation(previousOperator);
     c->endLayer();
 }
 
@@ -1671,10 +1669,8 @@ template<class T> void CanvasRenderingContext2D::fullCanvasCompositedStroke(cons
     GraphicsContext* c = drawingContext();
     ASSERT(c);
     c->beginLayer(1, state().m_globalComposite);
-    CompositeOperator previousOperator = c->compositeOperation();
     c->setCompositeOperation(CompositeSourceOver);
     strokePrimitive(area, c);
-    c->setCompositeOperation(previousOperator);
     c->endLayer();
 }
 
@@ -2236,10 +2232,8 @@ void CanvasRenderingContext2D::drawTextInternal(const String& text, float x, flo
 
     if (isFullCanvasCompositeMode(state().m_globalComposite)) {
         c->beginLayer(1, state().m_globalComposite);
-        CompositeOperator previousOperator = c->compositeOperation();
         c->setCompositeOperation(CompositeSourceOver);
         c->drawBidiText(font, textRunPaintInfo, location, Font::UseFallbackIfFontNotReady);
-        c->setCompositeOperation(previousOperator);
         c->endLayer();
         didDraw(clipBounds);
     } else if (state().m_globalComposite == CompositeCopy) {